a. Field of Invention
The invention relates to electronic device fabrication processes and, more particularly to a process for removing indium oxide from indium bumps used in flip chip hybridization (“bump bonding”).
b. Background of the Invention
Flip chip hybridization, also known as bump bonding or Controlled Collapse Chip Connection (C4), is an efficient method for connecting semiconductor devices using solder bumps deposited onto their contact pads. The semiconductor device is fabricated by forming integrated circuit(s) on a wafer. Contact pads are then metalized on the surface of the ICs, and solder dots or “bumps” are deposited on each of the pads. The ICs are then cut from the wafer to form “chips”. The chips are flipped and positioned so that the solder bumps are facing connectors on external circuitry. The solder bumps are then re-melted (typically using hot air) to complete the connections. This eliminates the need for wire bonding in which the chip is mounted upright and wires are used to interconnect the chip pads to external circuitry. This approach is particularly well-suited for flight applications as it significantly reduces the dimensions of the final device.
FIG. 1 is a front perspective view of solder bumps. The solder bumps are composed of indium metal deposited by thermal evaporation onto the pads. Indium bump technology has been a part of the electronic interconnect process field for many years. However, obtaining a reliable, high yield process for high density patterns of bumps can be quite difficult. One problem, in particular, is the tendency of the indium bumps to oxidize during exposure to air. The length of air exposure may vary during fabrication, and so there can be a great deal of variability in the amount of oxidation. As the level of oxidation increases, the contact resistance of the solder joint between the two halves of the flip-chip structure also increases, to the point where the bump may act as an open circuit preventing the assembled device from functioning correctly. Therefore, it is useful to have a reliable process that can remove this oxidized layer.
Several publications and patents have described methods where aqueous acidic solutions (e.g. HCl) have been used to etch away the indium oxide, leaving indium metal behind. See. For example, U.S. Pat. No. 4,865,245 to Schulte et al. issued Sep. 12, 1989, where devices are etched to remove oxide from the contact bumps and to prevent subsequent oxidation. Unfortunately, this approach is somewhat limited. Once the oxide is removed, HCl can continue to attack the indium, reducing the size of the bump. In addition, wet chemical processes are isotropic, meaning that they will attack the bump from all sides. This process can cause the bumps to be undercut.
FIG. 2 is a front perspective view of indium bumps of FIG. 1 after dipping in HCl. It is apparent in comparing FIGS. 1 and 2, severe corrosion (a) and undercut (b) is observed for the HCl treated bump of FIG. 2. Note that corrosion (a) and undercut (b) is only evident on indium bumps resting on an under-bump metallization (UBM) Ti/Pt metal bilayer (c). This undercutting results in a lack of mechanical stability prior to bonding. HCl can also attack the bond pad material, or under bump metallization layers (c), which, ironically, can create a new mechanism for open circuit conditions “opens” that the acidic treatment was meant to eliminate.
An ideal process for removing indium oxide from the indium bumps would reduce the oxide without attacking the indium metal. It is known that the use of a gas will not attack the indium metal, and toward this end hydrogen gas has been attempted.
FIG. 3 is a plot of the oxide reduction rates for various oxides when exposed to molecular hydrogen, as a function of temperature (degrees C.). The reduction rate (y-axis) is represented as decreasing thickness (nm) per minute. See, G. Humpston and D. M. Jacobson, Principles of Soldering Materials, Park, Ohio: ASM International, pp. 126-130 (2004). Unfortunately, from the center graph it can be seen that the reduction of indium oxide to indium metal in molecular hydrogen requires temperatures in excess of 380 degrees C. to achieve a reasonable reaction rate. These high temperatures are often not compatible with the types of devices that are typically hybridized.
The prior art suggests that indium tin oxide (ITO) can be etched selectively using CH4/H2 plasmas. See, I. Adesida et al., Etching Of Indium Tin Oxide In Methane/Hydrogen Plasmas, J. Vac. Sci. Technology B., Vol. 9, No. 6 (1991). This is due to the tendency of the oxygen in the ITO to form volatile COx species, with indium metal forming a volatile organometallic compound, In(CH3)3. Saia et al., Selective Reactive Ion Etching Of Indium-Tin Oxide In A Hydrocarbon Gas Mixture, J. Electrochemical Soc., vol. 138, no 2, pp. 493 (1991).
United States Patent Application 20030019917 by Furuno et al. published Jan. 30, 2003 teaches that atomic hydrogen generated in a plasma can be a potential alternative to molecular hydrogen due to its greater reactivity. However, a single-step plasma-based hydrogen process for removing indium oxide from the indium bumps as disclosed does not completely reduce the oxide. It would be greatly advantageous to provide a multi-step process for removing indium oxide from the indium bumps that is more effective in reducing the oxide, and yet does not require the use of halogens, does not change the bump morphology, does not attack the bond pad material or under-bump metallization layers, and creates no new mechanisms for open circuits.